Tunnel or tunneling field effect transistors (“TFET”) in the form of gated Esaki diodes (e.g., gated diodes having a negative resistance characteristics and/or operating in the reverse or Zener direction) are currently considered for digital applications operating at VDD≤0.3V. The main merit of these transistors is the possibility to reduce the off current at reduced gate swing using the steep slope where the transistors operate below the thermal limit of kT/q. The transistor operation relies on a band pass effect where the drive current is obtained based on tunneling across the bandgap (e.g., electrons tunneling from the conduction band of the n-type semiconductor material into a valence band of an adjacent p-type semiconductor material of a pn junction) that is controlled via the transistor gate. The off-current is reduced by the limited number of available states as the band gap is blocking direct tunneling of the carriers.
The main figures of merit for the tunnel transistor include the drive current, that is the current-level in the on-state, inverse sub-threshold slope (or sub-threshold swing) as well as the off-state current that defines how accurately the transistor may be turned off. The off-state is generally not an issue for a TFET since the off-state current is dictated by the reverse leakage current of the pn junction. Generally it is known to be difficult to obtain both a high drive current as well as a steep slope in the sub-threshold region. Part of the problem is related to the requirement of accurate alignment of the gate electrode with the pn-junction, where misalignment will reduce the gate effect and lower the electric field across the junction. High doping levels on either side of the junction will increase the drive currents but on the other hand the high doping levels will degrade the inverse sub-threshold slope because of the introduction of band tail states in the bandgap. An important aspect of steep slope tunnel devices is the amount of thermally excited carriers that take part in the tunneling; any potential pockets induced in the vicinity of the junction will introduce a thermal population of carriers (via the Fermi-Dirac function). If the tunneling is supplied by those carriers, the sub-threshold swing is immediately degraded to, at best, 60 mV/decade (the theoretical limit of thermally injected carriers). This is generally the case for TFET devices.
It is also known that the Esaki diodes preferably are fabricated in materials with small effective mass to increase the tunneling current and that heterostructures preferably are used to increase the drive current, one example being InAs/GaSb. Further problems are the effect of the D, that will increase the slope and in particular for heterostructure devices it is challenging to identify and to process high-κ dielectrics which are compatible with different semiconductor materials.